Not all of the update_site_option action hooks pass old value

[westi]

update_site_option has hooks similar to the ones in update_option but the add hooks don't pass the old value to the action:

	do_action( "update_site_option_{$option}", $option, $value );
	do_action( "update_site_option", $option, $value );

Unlike the ones in update_option:

		do_action( "update_option_{$option}", $oldvalue, $_newvalue );
		do_action( 'updated_option', $option, $oldvalue, $_newvalue );

[wpmuguru]

Looks good.

[nacin]

In [18521]:

Pass old value to update_site_option hooks. props westi, fixes #17974.